Electric motors are increasingly used on grass mowing machines for rotating cutting reels or other similar implements. On grass mowing machines, for example, power for multiple electric motors may be generated with a Lundell alternator that is belt driven by an internal combustion engine. A Lundell alternator can supply current under constant voltage conditions to an upper limit, depending on several factors including temperature. Above the upper limit, current can increase slightly, but voltage collapses sharply as the machine transforms from behaving as a voltage source to behaving as a current source. For example, an alternator that can supply 175 Amps at 48 Volts DC, can only support 35 Volts DC at 190 Amps. As a result, if current demand by the electric motors is very high, approaching or exceeding the capacity of the power generating component, the electrical power may be significantly lower due to a sharp voltage drop. Once the voltage drops too far, it can be insufficient to power other electrical functions on the machine, resulting in a condition commonly referred to as “brownout.” Other power generating devices such as permanent magnet alternators, electrochemical batteries, DC machines, and others suffer from overload problems of a type similar to the above. For example, electrochemical cells will undergo voltage collapse under high load conditions, particularly when the battery is highly discharged.
One condition demanding high current is start-up of multiple electric motors. For example, electric motors may have control systems that demand high current to start each reel spinning. The motors may be provided with control systems that typically include speed set points in the range of 2000 rpm to 3000 rpm. During start up, there is a large difference between actual reel speed and the set point, resulting in a proportional gain term that requires high current. The high current demand, especially during start-up of multiple electric motors that turn cutting reels, may exceed the capacity of the power generating component. As a result, a brownout condition may occur.
Another condition where high current is demanded is high frictional resistance. For example, multiple electric motors may demand high current if the cutting reels have high friction because of reel-to-bedknife interference or insufficient reel-to-bedknife clearance. Cutting reels that are rotated without a relief grind, or with interference, or less than the specified clearance (2×10−3 inches, for example), produce friction at the interface that heats both surfaces, causing both to expand and exacerbate the interference. The thermal expansion causes the current to increase. If there is enough interference between the reel and bedknife, the electric motors may eventually draw so much current that the voltage collapses. The result may be a brownout condition, in addition to possibly shutting down all of the cutting reels in the system.
A third condition that demands high current is excessive load. For example, a grass mowing machine may be subject to an excessive load when performing operations such as verticutting, or due to inadvertently scalping the turf or ground surface. These situations require high current that may brownout the alternator and shut down the electrical system.
It is desirable to operate multiple electric motors without losing functionality during start up, while encountering high friction, or when the motors are subject to high or excessive loads. It is desirable to operate multiple electric motors while maintaining bus voltage above a specified threshold. It is desirable to provide adequate power to multiple electrical motors without brownouts using power generated from a Lundell alternator, battery, or other electric power generating component.